The company says:
U.S. Dollars have a distinct, if somewhat evolving and ambiguous smell, with hints of inky cotton, leather wallets, metal cash drawers, and hundreds of sweaty palms, in a complexity that evolves and changes over time, as money circulates.
We used new, though not uncirculated, bills, focusing on the crispness of the fabric-laden paper and the ink. The result is a light, clean scent with enough of the ink to make it unmistakably money, in American bills.